The invention pertains to a signal generator circuit.
More specifically, the invention relates to a generator circuit for generating two jointly frequency-variable sinusoidal signals which are phase shifted with respect to one another by exactly 90.degree.. The generator circuit includes a fixed frequency oscillator whose output is connected to the input of a 90.degree. phase shifter and to the first input of an analog multiplier, and further includes a second analog multiplier whose first input is connected to the output of the 90.degree. phase shifter.
One such generator circuit is disclosed in the European patent specification No. B1-0 003 867. In this circuit a phase-locked loop which is locked to a stereo sound-modulated carrier signal, represents a fixed frequency oscillator which is connected to the first input of a first analog multiplier and via a 90.degree. phase shifter to the first input of a second analog multiplier. To the respective second input of the multipliers there is applied the stereo sound-modulated carrier signal. The two output signals of the multipliers represent two components of the stero signal differing from one another and are applied to parts of the circuit, which are of no particular interest in this case, for finally generating both the right-hand and the left-hand sound channel signal.
The invention is useful in the field of analog quadrature mixture which requires two sinusoidal signals which are phase shifted with respect to one another by 90.degree.. Very particular requirements are placed on such quadrature mixing circuits in the case of the direct demodulation of amplitude- or frequency-modulated carrier signals according to the "third method", also referred to as the weafer deep-IF or zero-IF method. Requirements are placed on the circuits with respect to the accuracy of the 90.degree. phase shift in the case of an additional frequency variability, because conversion to an intermediate frequency is omitted and the conversion is carried out directly from the carrier frequency range into the LF range, cf. Meinke/Gundlach, Taschenbuch der Hochfrequenztechnik, 2nd edition, 1962, pp. 1311 to 1313 and 1499. A 1% phase error which is likely to result from the frequency dependence of the 90.degree. phase shifter reduces e.g., according to the German published Patent Application No. DE-A-23 58 288, the signal-to-noise ratio 40 dB in the case of amplitude-modulated carriers. This has a disturbing effect in audio applications.